placoderm


placoderm
/plak"euh derrm'/, n.
any of various extinct jawed fishes of the class Placodermi, dominant in seas and rivers during the Devonian Period and characterized by bony armored plates on the head and upper trunk.
[1855-60; < NL Placodermi name of the class, pl. of PLACODERMUS, equiv. to placo- ( < Gk; see PLACOID, -O-) + -dermus -DERM]

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▪ fossil fish
      any member of an extinct group (Placodermi) of primitive jawed fishes known only from fossil remains. Placoderms existed throughout the Devonian period (416,000,000–359,000,000 years ago), but only two species persisted into the succeeding Carboniferous period. During the Devonian they were a dominant group, occurring in all continents except South America in a variety of marine and freshwater sediments.

      Most placoderms were small or moderate in size, but a few may have reached a length of 13 feet (4 metres). The name is derived from their characteristic armour of dermal, or skin, bones. This armour formed a head shield and a trunk shield, the two commonly connected by a paired joint in the neck region. The arrangement of bones is so different from that of modern fishes with bony skeletons that it is unlikely that the bones of the two groups are homologous (similar in origin).

      The earliest placoderms were heavily armoured and were bottom-dwelling. Many later forms became highly specialized for this way of life. Others became adapted for fast swimming between the surface and the bottom. Bottom-dwelling placoderms, such as the antiarchs, had small, ventrally placed mouths and presumably fed on bottom detritus and small invertebrates. Fossil remains indicate that some species had heavy, blunt jaw plates adapted for crushing hard-shelled invertebrates, while others were able to open their jaws wide enough to swallow smaller fish. Some placoderms, such as members of the genus Dunkleosteus, reached sizes of 10 metres (30 feet) or more and were the dominant predators of the Devonian seas.

      The origin of the placoderms is unknown, although it is possible that they may have shared a common ancestor with sharks, skates, and rays, as well as with true “bony” fishes.

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Universalium. 2010.

Look at other dictionaries:

  • placoderm — PLACODÉRM, placodermi, s.m. (La pl.) Subclasă de peşti fosili, paleozoici, cu scheletul cartilaginos şi cu corpul acoperit de plăci osoase; (şi la sg.) peşte din această subclasă. – Din fr. placodermes, germ. Plakoderm. Trimis de oprocopiuc,… …   Dicționar Român

  • Placoderm — Plac o*derm, n. [Gr. pla x, plako s, tablet + de rma skin.] (Paleon.) One of the Placodermi. [1913 Webster] …   The Collaborative International Dictionary of English

  • placoderm — noun Etymology: New Latin Placodermi, ultimately from Greek plak , plax + derma skin more at derm Date: circa 1865 any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures …   New Collegiate Dictionary

  • placoderm — noun A member of an extinct group of fish with armored heads; the group lived during the Silurian and Devonian periods …   Wiktionary

  • placoderm — pla|co|derm Mot Agut Nom masculí …   Diccionari Català-Català

  • placodérm — s. m., pl. placodérmi …   Romanian orthography

  • placoderm — [ plakə(ʊ)də:m] noun a fossil fish of the Devonian period, having the front part of the body encased in broad flat bony plates. Origin C19: from Gk plax, plak flat plate + derma skin …   English new terms dictionary

  • placoderm — plac·o·derm …   English syllables

  • placoderm — plac•o•derm [[t]ˈplæk əˌdɜrm[/t]] n. pal any of various extinct jawed fishes of the class Placodermi, chiefly of the Devonian Period: many were armored in bony plates • Etymology: 1855–60; < NL Placodermi name of the class, pl. of… …   From formal English to slang

  • placoderm — /ˈplækoʊdɜm/ (say plakohderm) noun a member of an extinct group of fish, thought to be ganoids, existing in the late Devonian period. {New Latin, from Greek plakos tablet + derma skin} …   Australian English dictionary


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